Semi - I had the same experience with my 45 cal LRH using the same bullet/sabot combo. But I was using Pyrodex P. Open sights too.... Anyway, I got good groups with 70 gr. at 50 yds. But when I went out to 80 yds, the group was still good but the POI was too high and I had the rear sight down as low as I could get it. When I boosted up the charge to 80 grs, it POI lowered and the group was tighter.
Can't explain why one load shot better at 50 yds and the other at 100 but I've seen wierder things happen.